Fundamentals of Distance
Protection
GE Multilin
Outline
•
•
•
•
•
•
•
•
Transmission line introduction
What is distance protection?
Non-pilot and pilot schemes
Redundancy considerations
Security for dual-breaker terminals
Out-of-step relaying
Single-pole tripping
Series-compensated lines
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March 18, 2016
Transmission Lines
A Vital Part of the Power System:
• Provide path to transfer power between generation and load
• Operate at voltage levels from 69kV to 765kV
• Deregulated markets, economic, environmental requirements
have pushed utilities to operate transmission lines close to their
limits.
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March 18, 2016
Transmission Lines
Classification of line length depends on:
 Source-to-line Impedance Ratio (SIR),
and
 Nominal voltage
Length considerations:
 Short Lines: SIR > 4
 Medium Lines: 0.5 < SIR < 4
 Long Lines: SIR < 0.5
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March 18, 2016
Typical Protection Schemes
Short Lines
• Current differential
• Phase comparison
• Permissive Overreach Transfer Trip (POTT)
• Directional Comparison Blocking (DCB)
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March 18, 2016
Typical Protection Schemes
Medium Lines
• Phase comparison
• Directional Comparison Blocking (DCB)
• Permissive Underreach Transfer Trip (PUTT)
• Permissive Overreach Transfer Trip (POTT)
• Unblocking
• Step Distance
• Step or coordinated overcurrent
• Inverse time overcurrent
• Current Differential
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March 18, 2016
Typical Protection Schemes
Long Lines
• Phase comparison
• Directional Comparison Blocking (DCB)
• Permissive Underreach Transfer Trip (PUTT)
• Permissive Overreach Transfer Trip (POTT)
• Unblocking
• Step Distance
• Step or coordinated overcurrent
• Current Differential
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March 18, 2016
What is distance protection?
Intended
REACH point
F1
Z
I*Z
V=I*ZF
I*Z - V
RELAY (V,I)
For internal faults:
> IZ – V and V approximately
in phase (mho)
> IZ – V and IZ approximately
in phase (reactance)
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March 18, 2016
What is distance protection?
F2
Intended
REACH point
Z
I*Z
V=I*ZF
I*Z - V
RELAY (V,I)
For external faults:
> IZ – V and V approximately
out of phase (mho)
> IZ – V and IZ approximately
out of phase (reactance)
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March 18, 2016
What is distance protection?
Intended
REACH point
Z
RELAY
10 /
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March 18, 2016
Source Impedance Ratio,
Accuracy & Speed
Relay
Lin
e
System
Voltage at the relay:
VR  VN
f LOC [ PU ]
f LOC [ PU ]  SIR
Consider SIR = 0.1
Fault location
Voltage
(%)
Voltage change
(%)
75%
88.24
2.76
90%
90.00
0.91
100%
90.91
N/A
110%
91.67
0.76
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March 18, 2016
Source Impedance Ratio,
Accuracy & Speed
Relay
System
Lin
e
Voltage at the relay:
VR  VN
f LOC [ PU ]
f LOC [ PU ]  SIR
Consider SIR = 30
Fault location
Voltage
(%)
Voltage change
(%)
75%
2.4390
0.7868
90%
2.9126
0.3132
100%
3.2258
N/A
110%
3.5370
0.3112
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March 18, 2016
Challenges in relay design
> Transients:
– High frequency
– DC offset in currents
– CVT transients in
voltages
30
voltage, V
20
High Voltage Line
C1
6
steady-state
output
10
0
-10
3
1
5
CVT output
C2
Secondary Voltage
Output
-20
2
7
-30
0
1
2
power cycles
3
4
4
8
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March 18, 2016
Challenges in relay design
> Transients:
– High frequency
– DC offset in currents
– CVT transients in
voltages
60
voltage, V
40
High Voltage Line
C1
6
steady-state
output
20
0
-20
3
1
CVT
output
5
C2
Secondary Voltage
Output
-40
2
7
-60
0
1
2
power cycles
3
4
4
8
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March 18, 2016
Challenges in relay design
100
vA
vB
100
vC
80
20
0
-20
-40
-60
-80
-100
-0.5
0
0.5
1
1.5
50
SPOL
Sorry… Future (unknown)
0
-50
5
iA
4
SOP
3
Current [A]
Voltage [V]
40
Reactance comparator [V]
60
-100
2
0
0.5
1
1.5
power cycles
1
0
iB, iC
-1
-2
-3
-0.5
-0.5
0
0.5
1
1.5
> In-phase = internal
fault
> Out-of-phase =
external fault
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March 18, 2016
Transient Overreach
• Fault current generally contains dc offset in
addition to ac power frequency component
• Ratio of dc to ac component of current
depends on instant in the cycle at which fault
occurred
• Rate of decay of dc offset depends on
system X/R
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March 18, 2016
Zone 1 and CVT Transients
Capacitive Voltage Transformers (CVTs) create certain
problems for fast distance relays applied to systems with
high Source Impedance Ratios (SIRs):
> CVT-induced transient voltage components may
assume large magnitudes (up to 30-40%) and last for
a comparatively long time (up to about 2 cycles)
> 60Hz voltage for faults at the relay reach point may be
as low as 3% for a SIR of 30
> the signal may be buried under noise
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March 18, 2016
Zone 1 and CVT Transients
CVT transients can cause distance relays to overreach.
Generally, transient overreach may be caused by:
> overestimation of the current (the magnitude of the
current as measured is larger than its actual value,
and consequently, the fault appears closer than it is
actually located),
> underestimation of the voltage (the magnitude of the
voltage as measured is lower than its actual value)
> combination of the above
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March 18, 2016
Distance Element Fundamentals
Z1
End Zone
XL
R
XC
15
34
42
Actual Fault
Location
44
Reactance [ohm]
10
30
dynamic mho
zone extended
for high SIRs
Line
Impedance
5
18
22
Trajectory
(msec)
0
26
-5
-10
-5
0
Resistance [ohm]
5
10
Impedance
locus
may pass
below the origin of the Z-plane this would call for a time delay
to obtain stability
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March 18, 2016
CVT Transient Overreach
Solutions
> apply delay (fixed or adaptable)
> reduce the reach
> adaptive techniques and better filtering
algorithms
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March 18, 2016
CVT Transients – Adaptive
Solution
> Optimize signal filtering:
– currents - max 3% error due to the dc component
– voltages - max 0.6% error due to CVT transients
> Adaptive double-reach approach
– filtering alone ensures maximum transient
overreach at the level of 1% (for SIRs up to 5) and
20% (for SIRs up to 30)
– to reduce the transient overreach even further an
adaptive double-reach zone 1 has been
implemented
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March 18, 2016
CVT Transients – Adaptive
Solution
The outer zone 1:
> is fixed at the actual reach
> applies certain security delay to cope with CVT transients
The inner zone 1:
> has its reach dynamically
controlled by the voltage
magnitude
> is instantaneous
X
Delayed
Trip
Instantaneous
Trip
R
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March 18, 2016
Desirable Distance Relay
Attributes
Filters:
> Prefiltering of currents to remove dc decaying transients
– Limit maximum transient overshoot (below 2%)
> Prefiltering of voltages to remove low frequency transients
caused by CVTs
– Limit transient overreach to less than 5% for an SIR of
30
> Accurate and fast frequency tracking algorithm
> Adaptive reach control for faults at reach points
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March 18, 2016
Distance Relay Operating Times
25 /
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March 18, 2016
Distance Relay Operating Times
35ms
25ms
30ms
20ms
15ms
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March 18, 2016
Distance Relay Operating Times
SLG faults
LL faults
3P faults
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March 18, 2016
Actual maximum reach curves
Relay 4
100
90
Relay 3
80
Maximum Rach [%]
70
60
50
40
Relay 2
30
20
Relay 1
10
0
0
5
10
15
SIR
20
25
30
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March 18, 2016
Maximum Torque Angle
• Angle at which mho element has maximum
reach
• Characteristics with smaller MTA will
accommodate larger amount of arc resistance
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March 18, 2016
Mho Characteristics
Traditional
Directional angle
“slammed”
Directional
angle lowered
and “slammed”
Both MHO and
directional angles
“slammed” (lens)
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March 18, 2016
Load Swings
+XL
+ = LOOKING INTO LINE
normally considered
forward
Load
Trajectory
Operate
No Operate area
area
Typical load characteristic
impedance
+R
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March 18, 2016
Load Swings
“Lenticular”
Characteristic
Load swing
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March 18, 2016
Load Encroachment Characteristic
The load encroachment element responds to positive
sequence voltage and current and can be used to
block phase distance and phase overcurrent
elements.
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March 18, 2016
Blinders
• Blinders limit the operation of distance relays
(quad or mho) to a narrow region that parallels
and encompasses the protected line
• Applied to long transmission lines, where
mho settings are large enough to pick up on
maximum load or minor system swings
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March 18, 2016
Quadrilateral Characteristics
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March 18, 2016
Quadrilateral Characteristics
Ground Resistance
(Conductor falls on ground)
R
Resultant impedance outside of
the mho operating region
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March 18, 2016
Distance Characteristics Summary
Mho
Lenticular
JX
Quadrilatera
l
R
Standard for phase
elements
Used for phase elements
with long heavily loaded
lines heavily loaded
Better coverage for
ground faults due
to resistance added
to return path
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March 18, 2016
Distance Element Polarization
The following polarization quantities are commonly
used in distance relays for determining directionality:
• Self-polarized
• Memory voltage
• Positive sequence voltage
• Quadrature voltage
• Leading phase voltage
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March 18, 2016
Memory Polarization
> Positive-sequence memorized voltage is used for
polarizing:
– Mho comparator (dynamic, expanding Mho)
– Negative-sequence directional comparator (Ground
Distance Mho and Quad)
– Zero-sequence directional comparator (Ground
Distance MHO and QUAD)
– Directional comparator (Phase Distance MHO and
QUAD)
> Memory duration is a common distance settings (all zones,
phase and ground, MHO and QUAD)
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March 18, 2016
Memory Polarization
Static MHO characteristic (memory not established or
expired)
jX
ZL
Dynamic MHO characteristic for a reverse fault
Dynamic MHO characteristic for a forward fau
Impedance During Close-up Faults
R
ZS
40 /
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March 18, 2016
Memory Polarization
jX
ZL
Static MHO characteristic (memory not established or
expired)
Dynamic MHO characteristic for a forward fault
RL
R
ZS
Memory Polarization…Improved Resistive
Coverage
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March 18, 2016
Choice of Polarization
• In order to provide flexibility modern distance
relays offer a choice with respect to
polarization of ground overcurrent direction
functions:
– Voltage polarization
– Current polarization
– Dual polarization
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March 18, 2016
Ground Directional Elements
> Pilot-aided schemes using ground mho distance relays
have inherently limited fault resistance coverage
> Ground directional over current protection using either
negative or zero sequence can be a useful supplement to
give more coverage for high resistance faults
> Directional discrimination based on the ground quantities is
fast:
– Accurate angular relations between the zero and
negative sequence quantities establish very quickly
because:
 During faults zero and negative-sequence
currents and voltages build up from very low
values (practically from zero)
 The pre-fault values do not bias the developing
fault components in any direction
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March 18, 2016
Distance Schemes
Pilot Aided
Schemes
Non-Pilot Aided
Schemes
(Step Distance)
Communication
between Distance
relays
No Communication
between Distance
Relays
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March 18, 2016
Step Distance Schemes
• Zone 1:
– Trips with no intentional time delay
– Underreaches to avoid unnecessary operation for faults
beyond remote terminal
– Typical reach setting range 80-90% of ZL
• Zone 2:
– Set to protect remainder of line
– Overreaches into adjacent line/equipment
– Minimum reach setting 120% of ZL
– Typically time delayed by 15-30 cycles
• Zone 3:
– Remote backup for relay/station failures at remote
terminal
– Reaches beyond Z2, load encroachment a consideration
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March 18, 2016
Step Distance Schemes
Local
Z1
Z1
Remote
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March 18, 2016
Step Distance Schemes
Local
End
Zone
Z1
End
Zone
Z1
Remote
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March 18, 2016
Step Distance Schemes
Local
Z1
Breaker
Tripped
Breaker
Closed
Z1
Remote
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March 18, 2016
Step Distance Schemes
Local
Z2 (time delayed)
Z1
Z1
Z2 (time delayed)
Remote
49 /
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March 18, 2016
Step Distance Schemes
…
Z3 (remote backup)
Z2 (time delayed)
Z1
50 /
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March 18, 2016
Step Distance Protection
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March 18, 2016
Distance Relay Coordination
Over Lap
Local Relay – Z2
Remote Relay – Z4
Local Relay
Remote Relay
Zone 2 PKP
Zone 4 PKP
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March 18, 2016
Need For Pilot Aided Schemes
Local
Relay
Remote Relay
Communication
Channel
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March 18, 2016
Pilot Communications Channels
• Distance-based pilot schemes traditionally utilize
simple on/off communications between relays, but
can also utilize peer-to-peer communications and
GOOSE messaging over digital channels
• Typical communications media include:
– Pilot-wire (50Hz, 60Hz, AT)
– Power line carrier
– Microwave
– Radio
– Optic fiber (directly connected or multiplexed
channels)
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March 18, 2016
Distance-based Pilot Protection
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March 18, 2016
Pilot-Aided Distance-Based Schemes
 DUTT – Direct Under-reaching Transfer Trip
 PUTT – Permissive Under-reaching Transfer
Trip
 POTT – Permissive Over-reaching Transfer Trip
 Hybrid POTT – Hybrid Permissive Overreaching Transfer Trip
 DCB – Directional Comparison Blocking
Scheme
 DCUB – Directional Comparison Unblocking
Scheme
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March 18, 2016
Direct Underreaching Transfer Trip
(DUTT)
• Requires only underreaching (RU) functions which
overlap in reach (Zone 1).
•Applied with FSK channel
– GUARD frequency transmitted during normal
conditions
– TRIP frequency when one RU function operates
• Scheme does not provide tripping for faults beyond
RU reach if remote breaker is open or channel is
inoperative.
• Dual pilot channels improve security
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March 18, 2016
DUTT Scheme
Zone 1
Bus
Bus
Line
Zone 1
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March 18, 2016
Permissive Underreaching
Transfer Trip (PUTT)
• Requires both under (RU) and overreaching
(RO) functions
• Identical to DUTT, with pilot tripping signal
supervised by RO (Zone 2)
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March 18, 2016
PUTT Scheme
Zone 2
Zone 1
To protect end of
line
Bus
Bus
Line
Zone 1
Zone 2
Rx PKP
Zone 2
Local Trip
&
OR
Zone 1
60 /
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March 18, 2016
Permissive Overreaching Transfer
Trip (POTT)
• Requires overreaching (RO) functions (Zone
2).
• Applied with FSK channel:
– GUARD frequency sent in stand-by
– TRIP frequency when one RO function
operates
• No trip for external faults if pilot channel is
inoperative
• Time-delayed tripping can be provided
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March 18, 2016
POTT Scheme
Zone 2
Zone 1
Bus
Bus
Line
Zone 1
Zone 2
(Z1)
Tx
Zone 1
(Z1)
OR
Rx
AND
Zone 2
Trip
Line
Breakers
t
o
62 /
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March 18, 2016
POTT Scheme
POTT – Permissive Over-reaching Transfer
Trip
End
Zone
Communication
Channel
63 /
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March 18, 2016
POTT Scheme
Local Relay
FWD IGND
Local Relay – Z2
Remote
Relay FWD
IGND
Remote Relay – Z2
TRIP
Communicatio
n Channel
POTT RX
Local Relay
ZONE 2 PKP
OR
Ground Dir OC Fwd
POTT TX
ZONE 2 PKP
Remote Relay
OR
Ground Dir OC Fwd
64 /
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March 18, 2016
POTT Scheme
POTT RX 2
POTT RX 3
POTT RX 4
Local Relay
Communications
Channel(s)
POTT RX 1
POTT TX 1 A to G
POTT TX 2 B to G
POTT TX 3 C to G
POTT TX 4 Multi Phase
Remote Relay
65 /
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March 18, 2016
POTT Scheme
Current reversal example
TRIP
Local Relay
Remote Relay
Timer
Start Communication
Timer
Expire
Channel
GND
GNDDIR
DIROC
OCFWD
REV
POTT RX
POTT TX
ZONE
2 OC
ORREV
GND
DIR
GND DIR OC FWD
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March 18, 2016
POTT Scheme
Echo example
Remote FWD
IGND
Open
Remote – Z2
OPEN
Communication
Channel
POTT RX
Local Relay
POTT TX
TRIP
POTT TX
POTT RX
Communication
Channel
Remote Relay
67 /
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March 18, 2016
Hybrid POTT
• Intended for three-terminal lines and weak
infeed conditions
• Echo feature adds security during weak
infeed conditions
• Reverse-looking distance and oc elements
used to identify external faults
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March 18, 2016
Hybrid POTT
Zone 2
Zone 1
Remote
Local
Weak
system
Bus
Bus
Line
Zone 1
Zone 4
Zone 2
69 /
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March 18, 2016
Directional Comparison Blocking
(DCB)
• Requires overreaching (RO) tripping and blocking
(B) functions
• ON/OFF pilot channel typically used (i.e., PLC)
– Transmitter is keyed to ON state when blocking
function(s) operate
– Receipt of signal from remote end blocks
tripping relays
• Tripping function set with Zone 2 reach or greater
• Blocking functions include Zone 3 reverse and lowset ground overcurrent elements
70 /
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March 18, 2016
DCB Scheme
Zone 2
Zone 1
Remote
Local
Bus
Bus
Line
Zone 1
Zone 2
71 /
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March 18, 2016
Directional Comparison Blocking
(DCB)
End Zone
Communication Channel
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March 18, 2016
Directional Comparison Blocking
(DCB)
Internal Faults
Local Relay – Z2
FWD IGND
TRIP Timer
Start
Expired
TRIP
Zone 2 PKP
OR
NO
Local Relay GND DIR OC Fwd
Dir Block RX
Remote Relay
73 /
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March 18, 2016
Directional Comparison Blocking
(DCB)
External Faults
Local Relay – Z2
FWD IGND
Remote Relay – Z4
TRIP Timer
Start
No TRIP
REV IGND
Dir Block RX
Local Relay
Zone 2 PKP
OR
DIR BLOCK TX
Communication
Channel
GND DIR OC Fwd
Zone 4 PKP
Remote Relay
OR
GND DIR OC Rev
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March 18, 2016
Directional Comparison
Unblocking (DCUB)
• Applied to Permissive Overreaching (POR)
schemes to overcome the possibility of carrier signal
attenuation or loss as a result of the fault
• Unblocking provided in the receiver when signal is
lost:
– If signal is lost due to fault, at least one
permissive RO functions will be picked up
– Unblocking logic produces short-duration TRIP
signal (150-300 ms). If RO function not picked
up, channel lockout occurs until GUARD signal
returns
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March 18, 2016
DCUB Scheme
Forward
Bus
Bus
Line
Forward
(Un-Block)
Tx1
(Block)
Tx2
Trip
Line
Breakers
Forward
(Block)
Rx2
AND
AND
(Un-Block)
t
AND
o
AND
Rx1
Lockout
76 /
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March 18, 2016
Directional Comparison Unblocking
(DCUB)
End Zone
Communication Channel
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March 18, 2016
Directional Comparison Unblocking
(DCUB)
Normal conditions
Load Current
FSK Carrier
GUARD1 RX
FSK Carrier
GUARD1 TX
Local Relay
NO Loss of Guard
NO Permission
Remote Relay
GUARD2 TX
Communication
Channel
GUARD2 RX
NO Loss of Guard
NO Permission
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March 18, 2016
Directional Comparison Unblocking
(DCUB)
Normal conditions, channel failure
Load Current
Loss of Channel
FSK Carrier
GUARD1
NO
RX RX
FSK Carrier
GUARD1 TX
Local Relay
Loss of Guard
Block Timer Started
Expired
Block DCUB
until Guard OK
Remote Relay
GUARD2 TX
Communication
Channel
GUARD2
NO
RX RX
Loss of Guard
Block Timer Expired
Started
Block DCUB 79 /
until Guard
OKGE /
March 18, 2016
Directional Comparison Unblocking
(DCUB)
Internal fault, healthy
channel
Local Relay
– Z2
Remote Relay – Z2
TRIP
TRIP Z1
FSK Carrier
Local Relay
Zone 2 PKP
FSK Carrier
GUARD1
TRIP1
RXRX
GUARD1
TRIP1 TX
TX
GUARD2
TRIP2
TX TX
GUARD2
TRIP2
RXRX
Remote Relay
ZONE 2 PKP
Loss of Guard
Permission
Communication
Channel
80 /
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March 18, 2016
Directional Comparison Unblocking
(DCUB)
Internal fault, channel
failure
Local Relay
– Z2
Remote Relay – Z2
Loss of Channel
TRIP
TRIP Z1
FSK Carrier
Local Relay
Zone 2 PKP
FSK Carrier
GUARD1
NO
RX RX
GUARD1
TRIP1 TX
TX
GUARD2
TRIP2
TX TX
GUARD2
NO
RX RX
Loss of Guard
Block Timer Started
Duration Timer Started
Expired
Remote Relay
ZONE 2 PKP
Loss of Guard
Communication
Channel
81 /
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March 18, 2016
Redundancy Considerations
• Redundant protection systems increase dependability of the
system:
 Multiple sets of protection using same protection principle
and multiple pilot channels overcome individual element
failure, or
 Multiple sets of protection using different protection
principles and multiple channels protects against failure of
one of the protection methods.
• Security can be improved using “voting” schemes (i.e., 2-outof-3), potentially at expense of dependability.
• Redundancy of instrument transformers, battery systems, trip
coil circuits, etc. also need to be considered.
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March 18, 2016
Redundant Communications
End Zone
AND Channels:
POTT Less Reliable
DCB Less Secure
OR Channels:
Communication Channel 1
Communication Channel 2
More Channel Security
POTT More Reliable
DCB More Secure
More Channel Dependability
Loss of Channel 2
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March 18, 2016
Redundant Pilot Schemes
84 /
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March 18, 2016
Pilot Relay Desirable Attributes
• Integrated functions:
weak infeed
echo
line pick-up (SOTF)
• Basic protection elements used to key the
communication:
distance elements
fast and sensitive ground (zero and negative
sequence) directional IOCs with current,
voltage, and/or dual polarization
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March 18, 2016
Pilot Relay Desirable Attributes
Pre-programmed distance-based pilot schemes:
 Direct Under-reaching Transfer Trip (DUTT)
 Permissive Under-reaching Transfer Trip (PUTT)
 Permissive Overreaching Transfer Trip (POTT)
 Hybrid Permissive Overreaching Transfer Trip (HYB
POTT)
 Blocking scheme (DCB)
 Unblocking scheme (DCUB)
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March 18, 2016
Security for dual-breaker terminals
• Breaker-and-a-half and ring bus terminals are
common designs for transmission lines.
• Standard practice has been to:
– sum currents from each circuit breaker
externally by paralleling the CTs
– use external sum as the line current for
protective relays
• For some close-in external fault events, poor CT
performance may lead to improper operation of line
relays.
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March 18, 2016
Security for dual-breaker terminals
Accurate CTs preserve the
reverse current direction
under weak remote infeed
88 /
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March 18, 2016
Security for dual-breaker terminals
Saturation of CT1 may
invert the line current as
measured from externally
summated CTs
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March 18, 2016
Security for dual-breaker terminals
• Direct measurement of currents
from both circuit breakers allows
the use of supervisory logic to
prevent distance and directional
overcurrent elements from
operating incorrectly due to CT
errors during reverse faults.
• Additional benefits of direct
measurement of currents:
 independent BF protection
for each circuit breaker
 independent autoreclosing
for each breaker
90 /
GE /
March 18, 2016
Security for dual-breaker terminals
Supervisory logic should:
– not affect speed or sensitivity of protection elements
– correctly allow tripping during evolving external-tointernal fault conditions
– determine direction of current flow through each
breaker independently:
• Both currents in FWD direction  internal fault
• One current FWD, one current REV  external fault
– allow tripping during all forward/internal faults
– block tripping during all reverse/external faults
– initially block tripping during evolving external-tointernal faults until second fault appears in forward
direction. Block is then lifted to permit tripping.
91 /
GE /
March 18, 2016
Single-pole Tripping
• Distance relay must correctly identify a SLG
fault and trip only the circuit breaker pole for
the faulted phase.
• Autoreclosing and breaker failure functions
must be initiated correctly on the fault event
• Security must be maintained on the healthy
phases during the open pole condition and any
reclosing attempt.
92 /
GE /
March 18, 2016
Out-of-Step Condition
• For certain operating conditions, a severe
system disturbance can cause system
instability and result in loss of synchronism
between different generating units on an
interconnected system.
93 /
GE /
March 18, 2016
Out-of-Step Relaying
Out-of-step blocking relays
– Operate in conjunction with mho tripping relays
to prevent a terminal from tripping during severe
system swings & out-of-step conditions.
– Prevent system from separating in an
indiscriminate manner.
Out-of-step tripping relays
– Operate independently of other devices to
detect out-of-step condition during the first pole
slip.
– Initiate tripping of breakers that separate system
in order to balance load with available
generation on any isolated part of the system.
94 /
GE /
March 18, 2016
Out-of-Step Tripping
When the inner
characteristic is
entered the element
is ready to trip
The locus must stay
for some time
between the outer
and middle
characteristics
Must move and stay
between the middle
and inner
characteristics
95 /
GE /
March 18, 2016
Power Swing Blocking
Applications:
> Establish a blocking signal for stable power swings (Power
Swing Blocking)
> Establish a tripping signal for unstable power swings (Outof-Step Tripping)
Responds to:
> Positive-sequence voltage and current
96 /
GE /
March 18, 2016
Series-compensated lines
Benefits of series capacitors:
• Reduction of overall XL of long lines
• Improvement of stability margins
• Ability to adjust line load levels
• Loss reduction
• Reduction of voltage drop during severe disturbances
• Normally economical for line lengths > 200 miles
E
Xs
SC
XL
Infinte
Bus
97 /
GE /
March 18, 2016
Series-compensated lines
SCs create unfavorable conditions for protective relays and
fault locators:
• Overreaching of distance elements
• Failure of distance element to pick up on low-current faults
• Phase selection problems in single-pole tripping
applications
• Large fault location errors
E
Xs
SC
XL
Infinte
Bus
98 /
GE /
March 18, 2016
Series-compensated lines
Series Capacitor with MOV
99 /
GE /
March 18, 2016
Series-compensated lines
100 /
GE /
March 18, 2016
Series-compensated lines
Dynamic Reach Control
101 /
GE /
March 18, 2016
Series-compensated lines
Dynamic Reach Control for External Faults
102 /
GE /
March 18, 2016
Series-compensated lines
Dynamic Reach Control for External Faults
103 /
GE /
March 18, 2016
Series-compensated lines
Dynamic Reach Control for Internal Faults
104 /
GE /
March 18, 2016
Distance Protection Looking
Through a Transformer
• Phase distance elements can be set to see beyond
any 3-phase power transformer
• CTs & VTs may be located independently on
different sides of the transformer
• Given distance zone is defined by VT location (not
CTs)
• Reach setting is in sec, and must take into
account location & ratios of VTs, CTs and voltage
ratio of the involved power transformer
105 /
GE /
March 18, 2016
Transformer Group Compensation
Depending on location of VTs and CTs, distance relays need to
compensate for the phase shift and magnitude change caused by the
106 /
power transformer
GE /
March 18, 2016
Setting Rules
• Transformer positive sequence impedance must be
included in reach setting only if transformer lies
between VTs and intended reach point
• Currents require compensation only if transformer
located between CTs and intended reach point
• Voltages require compensation only if transformer
located between VTs and intended reach point
• Compensation set based on transformer connection
& vector group as seen from CTs/VTs toward reach
point
107 /
GE /
March 18, 2016
Distance Relay Desirable
Attributes
> Multiple reversible distance zones
> Individual per-zone, per-element characteristic:
– Dynamic voltage memory polarization
– Various characteristics, including mho, quad,
lenticular
> Individual per-zone, per-element current supervision
(FD)
> Multi-input phase comparator:
– additional ground directional supervision
– dynamic reactance supervision
> Transient overreach filtering/control
> Phase shift & magnitude compensation for distance
applications with power transformers
108 /
GE /
March 18, 2016
Distance Relay Desirable
Attributes
> For improved flexibility, it is desirable to have the following
parameters settable on a per zone basis:
– Zero-sequence compensation
– Mutual zero-sequence compensation
– Maximum torque angle
– Blinders
– Directional angle
– Comparator limit angles (for lenticular characteristic)
– Overcurrent supervision
109 /
GE /
March 18, 2016
Distance Relay Desirable
Attributes
> Additional functions
– Overcurrent elements (phase, neutral, ground,
directional, negative sequence, etc.)
– Breaker failure
– Automatic reclosing (single & three-pole)
– Sync check
– Under/over voltage elements
> Special functions
– Power swing detection
– Load encroachment
– Pilot schemes
110 /
GE /
March 18, 2016
111 /
GE /
March 18, 2016